1. Field of the Disclosure
This disclosure generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to tilted toroidal coil near the drillbit conveyed in the borehole for introducing and measuring individual survey currents injected into the wall of the borehole.
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, called “galvanic” devices, electrodes emit current into the earth formations in order to determine resistivity. One of the simplest forms of galvanic devices is the so-called “normal” device where a current electrode emits a current through the earth formation to a remote return location, and a voltage electrode measures the potential due to that current with respect to the remote reference location. In the second category, inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. Resistivity can be determined by measuring either the magnitude or the attenuation caused by propagation of this current by means of the same antenna or separate receiver antennas. Various illustrative embodiments of the present disclosure belong to the first category, galvanic devices, as described in more detail below.
Normal type devices have been used extensively in the logging industry to determine formation resistivity when the borehole fluid is either more resistive or is slightly less resistive than the formation. One of the earlier devices was that of Doll wherein a so-called “micronormal” tool was used for measuring resistivity near the borehole wall.
Some galvanic devices are designed to focus the survey current into the formation whose resistivity is to be determined. For example, Birdwell (U.S. Pat. No. 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Other galvanic devices focus the current near the borehole wall. This is useful when the mud resistivity is much lower than the formation resistivity.
U.S. Pat. No. 6,050,068 to Chemali et al., having the same assignee as the present disclosure, the contents of which are incorporated herein by reference, teaches an apparatus for making measurements of the resistivity of an earth formation during the drilling process. A drill bit is conveyed on a bottom assembly for the purpose of drilling the borehole. At least one measure electrode is carried on an exterior surface of the drillbit and is used to convey a measure current into the formation. A device set in a cavity of the drillbit is responsive to at least one of (i) the current, and, (ii) the voltage of the at least one measure electrode. The response of this device indicates a measurement of resistivity of the formation near the borehole. A source of voltage coupled to the at least one electrode provides a specified voltage to the at least one electrode. The device disclosed in Chemali is an unfocused device. Consequently, there may be variations in the measured resistivity due to changes in standoff of the electrode during rotation of the tool. It may be desirable to reduce the effect of the standoff. Those versed in the art and having benefit of the present disclosure would recognize that standoff at the bit may be caused by bouncing of the bit during drilling.
In many drilling applications it is necessary to stop the drilling process before or shortly after the bit penetrates a new formation. In order to determine the position for stopping, the drilling industry uses resistivity tools which are sensitive at or near the bit. Typically, these tools use a toroidal coil for transmitting a current along the drillstring and a receiver toroidal coil for measuring the current near the bit in direction of the borehole.
The Bit Resistivity Tool offers a service which is known as “geostopping,” in which the BHA stops before a specific formation (e.g. reservoir) is penetrated. Geostopping can be useful when a casing has to be placed directly at the beginning of a reservoir, or when the bit has to be exchanged because the new formation is expected to be harder, etc. Therefore, it is important to offer a sensor which is able to measure resistivity values at the bit or ahead of the bit.
A transmitting toroidal coil antenna disposed on an electrically conductive metal body, such as a drill collar on a drill string, has been used in the past. The transmitting antenna is energized to induce a current which travels in a path that includes the body and the formations. An electrode is disposed on the body, and an electrical signal resulting from said current is measured at the electrode to obtain an indication of the resistivity of the formations.
These tools provide a best response if the new formation is penetrated perpendicularly. In this case, there will be a very quick response curve with a very steep slope. If a high inclination of the BHA to the formation is present, the response deteriorates; the curve slope is flat and it is not easy to determine the point of penetration. Unfortunately penetrations in which there is a very small angle between borehole and formation are more frequent in present drilling practices. Therefore, there is a need to recognize the beginning penetration of a new formation in situations where the angle between borehole and formation border is very small. The following invention addresses this need.